This invention relates to an exothermic alloying fuse assembly and to a solid electrolytic capacitor package for forming a fused solid electrolyte capacitor assembly.
Miniature size solid electrolytic capacitors are used on circuit boards in large numbers by the electronics industry in the manufacture of computers. If a solid electrolytic capacitor on a circuit board should prove to be defective and overheat it is essential that the current flow to such capacitor be interrupted to prevent the temperature of the capacitor from rising to a dangerously high level which can result in the ignition and burning of the capacitor and ultimately in the destruction of the device incorporating the faulty capacitor. Accordingly, it is common practice to attach a fusing element to a capacitor lead, preferably within the capacitor housing itself, which actuates in response to the electrical current passed by the failing capacitor and induces an open circuit.
Most conventional fusing elements consist of a low melting metal member placed in series with a capacitor terminal and its corresponding lead. The metal member is adapted to melt at a predetermined current level which should, at least in theory, open up the electrical circuit and isolate the capacitor. In practice however, if the fusing element is embodied within the capacitor, as is preferred, the encapsulating material holds the element in situ even after it is melted which still allows current to flow. Accordingly, the reliability of a low melting temperature metal for use as a fusible link in a capacitor assembly is very poor.
Alloying metals which alloy exothermically and rapidly when brought to their kindling temperature have been used as a substitute for the low melting metal fuse in constructing a fused capacitor. A typical alloying material of this type consists of a bimetallic composite of aluminum and a precious metal such as palladium. The electrical current required to heat the fuse resistively to the kindling temperature is determined by the effective length of the alloying material and its cross sectional diameter. Unfortunately, when incorporated into the body of a miniature size solid electrolytic capacitor the bimetallic fuse may be equally as unreliable as the low melting metal fuse. The exothermic reaction of the bimetallic fuse results in very high temperatures which causes the resinous encapsulant surrounding the bimetallic fusing element to char. The charred material forms a carbonaceous deposit which supports arcing and to a more limited extent current flow. In addition, assembly of a bimetallic alloying fuse in a miniature solid electrolytic capacitor has proven to be extremely difficult because of its fragile nature. The fusing bimetallic element must be very small in diameter, generally between 1-5 mils in thickness to provide the desired low electrical current to heat the wire resistively to its kindling temperature. Accordingly, it is easy to break and difficult to work with. Moreover, the joining of the ends of a bimetallic fuse to a capacitor lead so as to complete an electrical circuit must be done with great care to avoid initiating the exothermic reaction.
It is therefore desired to provide an exothermic alloying fuse assembly which is easily incorporated into an electrical device such as a solid electrolytic capacitor and which will not leave a carbon residue after ignition to support further conduction.